<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="js/jquery-3.3.1.min.js"></script>
</head>
<body>
    <div id="hello">my div</div>

    <p id="world">my p</p>

    <div>嘿嘿</div>
</body>
</html>

<script>
    /**
     * DOM对象转jQuery对象
     */ 
    var objDom = document.getElementById('hello')
    var objJquery = $(objDom)
    // console.log(objJquery.text())
    objJquery.css('background', 'pink')

    /**
     * jQuery对象转DOM对象
     */
    var jqueryObj = $('#world')
    // var domObj = jqueryObj[0]
    var domObj = jqueryObj.get(0)
    domObj.style.background = 'green'

    /**
     * 选择器
     */ 
    var objs = $('div')
    console.log(objs)
</script>